1. Field of the Invention
The present invention relates to the field of photodetectors. Embodiments of the present invention provide room temperature IR, visible and UV photodetectors that incorporate wavefunction engineering and/or photomodulated electron tunneling. They are made by electrochemical self-assembly of nanowires. Photodetectors of embodiments of the invention have an infrared (wavelength averaged) detectivity exceeding 107 cm-(√Hz)/watt at room temperature.
2. Description of Related Art
A photodetector is a device that senses light by producing an electrical current under illumination. The onset of this current signals the presence of light. Infrared (IR) photodetectors are widely used in car collision avoidance systems, detection of buried mines, night vision, forensics, search for water in the moon and extra-solar planets, formation of stellar nurseries, missile defense, and analytical chemistry to name a few. Ultra-violet (UV) photodetectors are used for fire and explosion detection, UV exposure detection, chemical-related chromatography applications, such as High Performance Liquid Chromatography (HPLC), astronomy and missile defense. Visible light detectors are used in forensics and analytical chemistry.
The best photodetectors are typically made of semiconductors. They work by absorbing an incident photon that excites an electron from the valence band (Ev) to the conduction band (Ec) of the semiconductor leaving behind a hole (electron vacancy) in the valence band, as shown in FIG. 1. Both the electron and the hole can carry current. Thus, the absorption of the photon generates an electron-hole pair that causes an additional amount of current to flow through the detector if a potential bias is applied across it. This additional current signals the presence of the photon, thereby effectively “detecting” it. The only requirement for this mechanism to work is that the photon's energy hf (h=Planck constant and f=photon frequency) must exceed the energy separation between the conduction and the valence band, which is the semiconductor's bandgap ΔE:hf≧ΔE  EQ. (1)This condition is mandated by the principle of energy conservation. Because of the requirement in Equation (1), IR detectors will need semiconductors of very small bandgap ΔE because IR photons have very small energies hf (typically a few milli-electron volts, or meV).
Typical IR detectors are indeed made of small-bandgap semiconductors like InSb and HgCdTe. However, if the semiconductor has a small bandgap, then phonons (quanta of lattice vibrations) can also excite electrons from the valence to the conduction band, since phonons have energies comparable to those of IR photons, and satisfy Equation (1). Therefore, current can flow through the detector even in the dark because of the phonons exciting electrons from the valence to the conduction band. The population of phonons increases rapidly with temperature because phonons obey Bose-Einstein statistics. At room temperature, the phonon population in a detector may vastly exceed the photon population in incident IR light. In that case, phonons would have already generated a large number of electron-hole pairs in the dark and the few additional pairs generated by IR light will make little difference to the pair population or the total current. In other words, the current under illumination by IR light Ilight will only slightly exceed the dark current Idark. That will make the signal-to-noise ratio (SNR) of the detector only slightly larger than unity:
                    SNR        =                                            I              light                                      I              dark                                ≅          1.                                    EQ        .                                  ⁢                  (          2          )                    
A low signal-to-noise ratio severely impairs the detection ability of the detector since there is hardly any difference between the current flowing in the dark and current flowing under illumination. Therefore, it is imperative to increase the room-temperature signal-to-noise ratio and make it considerably exceed unity by clever engineering.
The easiest way to increase the signal to noise ratio is to cool the detectors with cryogens, which will reduce the phonon population and the dark current. However, there is a strong need for semiconductor infrared (IR) photodetectors that work at room temperature without cooling and yet has large signal-to-noise ratio because of the cost and inconvenience of cryogenic or Peltier cooling. Room temperature IR detectors with adequate signal-to-noise ratio are usually fashioned out of microbolometers, thermopiles or pyroelectric detectors, but they are expensive and fragile. See P. W. Kruse and D. D. Skatrud, Uncooled Infrared Imaging Arrays and Systems, (Academic, San Diego, 1997).
Rugged room-temperature IR detectors have been derived from semiconductor structures fabricated with expensive molecular beam epitaxy. See A. Rakovska, V. Berger, X. Marcadet, B. Vinter, G. Glastre, T. Oksenhendler and D. Kaplan, Appl. Phys. Lett., 77, 397 (2000); and P. V. V. Jayaweera, S. G. Matsik, A. G. U. Perera, H. C. Liu, M. Buchanan and Z. R. Wasilewski, Appl. Phys. Lett., 93, 021105 (2008). Such fabrication methods, however, are not amenable to high throughput and therefore not commercially viable. Examples of conventional detectors include those disclosed in U.S. Pat. Nos. 8,313,972, 8,203,195, 7,989,753, 7,906,361, and 7,501,611, and in US Published Patent Application Nos. 20120138901 and 20080178924. They are typically fabricated with expensive techniques that have slow throughput.
There is a recent report that millions of IR detectors will be required across the planet to monitor the effects of global warming. Such detectors must operate without cooling and must be robust, reliable, rugged, cheap and mass-producible with high throughput. Present detectors cannot meet this requirement because they are either too fragile, or too expensive, or do not have sufficient signal-to-noise ratio. The invented detector meets this requirement since it works at room temperature and is robust, rugged, cheap and mass-producible.
In addition to the low signal-to-noise ratio at room temperature, a serious shortcoming of most semiconductor photodetectors is that they are not frequency-selective, i.e. they cannot lock on to a specific frequency of light at the exclusion of all other frequencies. Thus, they cannot satisfy the requirement of frequency-resolved analysis of radiation which is important in astronomy, forensics and molecular spectroscopy. However, nanowire based photodetectors are naturally frequency-selective since they absorb (and therefore detect) light within a narrow window of frequency. This happens because the absorption spectrum of nanowires exhibit distinct peaks at specific frequencies and these frequencies can be tailored with appropriate choice of nanowire materials. The peaks are caused by the fact that electrons and holes in nanowires are constrained to move in only one dimension (along the length of the nanowire) and cannot move sideways. This is known as quasi one-dimensional carrier confinement. One-dimensional confinement leads to frequency-selectivity of absorption and hence detection.
Synthesizing nanowires that exhibit absorption peaks (and hence frequency selectivity) is remarkably difficult since the nanowire diameter has to be smaller than the thermal DeBroglie wavelength of electrons, which may be a few nanometers. Nanowires have been made by various processes, including such processes as disclosed in U.S. Pat. Nos. 7,892,956, 7,691,201, 7,651,944, 7,635,600, 7,608,902, and 7,566,435 and in US Published Patent Application Nos. 20110108803, 20100239488, 20100147674, 20100002324, and 20070155173. Not all of them exhibit absorption peaks and hence not all of them can be fashioned into frequency selective photodetectors.
Nanowires prepared by the “porous alumina template” technique where semiconductors are selectively electrodeposited in nanometer sized pores of anodic alumina films produced by anodizing an aluminum foil in oxalic or sulfuric acid [see C. R. Martin, Science, 266 (1994) 1961; and S. Bandyopadhyay, et al., Nanotechnology 7 (1996) 360] are desirable since the fabrication process is inexpensive and have a high throughput. However, such nanowires are usually polycrystalline, have a high density of surface states (see V. Pokalyakin, et al., J. Appl. Phys. 97 (2005) 124306) and are replete with impurities absorbed during synthesis. Consequently, they do not commonly exhibit absorption peaks. Current reports in the literature reveal that CdS and CdSe nanowires produced by this technique exhibit blue shifts in their absorption spectra, but the absorption strengths increase monotonically with photon energy (see S. P. Mondal, K. Das, A. Dhar, S. K. Ray, Nanotechnology 18 (2007) 095606; and S. P. Mondal, K. Das, A. Dhar, S. K. Ray, in: Proceedings of the International Workshop on Physics of Semiconductor Devices, Mumbai, India, 2007, p. 855), instead of exhibiting peaks. Such nanowires therefore cannot be fashioned into frequency-selective detectors. The invented electrochemically synthesized nanowires of CdSe and ZnS, exhibit absorption peaks and hence frequency selectivity at room temperature.
In order to devise an IR detector that does not require cooling and yet has an adequate signal-to-noise ratio (SNR), a new approach to photodetection was adopted. It is based on the following premise. The rate of transition of electrons from one energy-level E1 to another E2 in any material, due to either photons or phonons, is given by Fermi's Golden Rule:
                                          S            ⁡                          (                                                E                  1                                ,                                  E                  2                                            )                                =                                                    2                ⁢                π                            ℏ                        ⁢                                                                            M                                                            E                      1                                        ,                                          E                      2                                                                                                  2                        ⁢                          δ              ⁡                              (                                                      E                    2                                    -                                      E                    1                                    -                  ℏω                                )                                                    ,                            EQ        .                                  ⁢                  (          3          )                    where E1 is the lower energy level, E2 is the higher energy level, and ME1,E2 is the matrix element for transition between the two levels, which is a measure of the strength of coupling between electrons and either photons or phonons. Coupling induces the transition from level E1 to E2.
Matrix element for electron-photon coupling is given by:Melec-photon∝Nphoton|∫ψ*2(r)eik·r[−iℏev·∇ψ1(r)]3r|2,  (4)where Nphoton is the photon occupation number that depends on the incident light intensity, ev is the unit vector in the direction of light polarization, k is the photon's wavevector, ψ1(r) is the initial state wavefunction in level E1 and ψ2(r) is the final state wavefunction in level E2. On the other hand, the electron-phonon coupling strength is:Melec-photon∝Nphonon|∫ψ*2(r)eik·rψ1(r)d3r|2,  EQ. (5)where Nphonon is the phonon occupation number. At equilibrium, it is given by the Bose-Einstein factor
      (                  N        phonon            =              1        /                  [                                    exp              ⁡                              (                )                                      -            1                    ]                      )    .Normally, Nphonon>>Nphoton at room temperature, which makes the phonon induced transitions much stronger than the photon induced ones. Consequently, Ilight−Idark<<Idark and the SNR is only slightly larger than unity.
However, if the electron's initial state (level E1) is a trap state where the electron is trapped and has a very localized wavefunction ψ1(r), as shown in FIG. 2, then the quantity ev·∇ψ1(r), which is the spatial derivative of ψ1(r) in the direction of light polarization, becomes very large. This happens regardless of the direction of light polarization, since the wavefunction is localized in all directions in three-dimensional space and is peaked at the trap site. Note also that eik·r≈1 since the spatial extent of the electron's wavefunction in the trap site is much smaller than the wavelength of IR light. On the other hand, the final state wavefunction ψ2(r) in the conduction band, where the electron is free, is delocalized and spread out in space (see FIG. 2). Hence, clearly if electrons are excited from trap states into the conduction band, as opposed to excitation from the valence band to the conduction band, then the integral in the photon coupling term in Equation (4) becomes relatively large because of the large spatial gradient of the initial state wavefunction, but the integral in the phonon coupling term in Equation (5) remains small because the spatial overlap between the spread-out final state wavefunction and the peaked initial state wavefunction is small. That could make Melec-photon at least comparable to Melec-phonon, even when Nphonon>>Nphoton. This will make Ilight and Idark perceptibly different at room temperature and provide an adequate SNR. Thus, “wavefunction engineering,” whereby the initial wavefunction is localized in space and the final wavefunction is delocalized, may result in acceptable SNR at room temperature.
In order to exploit wavefunction engineering, an IR detector is made out of a wide bandgap semiconductor whose bandgap energy is much larger than the energy of IR photons. Thus, neither photons not phonons can excite electrons directly from the valence band to the conduction band and cause current by generating electron-hole pairs. However, if the semiconductor is fashioned into a nanowire embedded in an insulating matrix, then the interface defects cause shallow trap states to form close to the conduction band in energy. The energy difference between the conduction band and the trap states is less than the energy of IR photons and phonons, so that both can excite electrons from the trap states into the conduction band. The electron in traps states cannot produce current since they are not free to move, but electrons excited into the conduction band can since they become free. Since the wavefunction in the traps state is localized but the wavefunction in the conduction band is delocalized, the matrix element for electron photon coupling becomes comparable to that of electron phonon coupling despite the fact that the phonons vastly outnumber the photons. Therefore, about equal numbers of electrons are excited from trap states into the conduction band by photons and phonons at room temperature when there are many more phonons around than photons. The current produced by the photons and that produced by the phonons are very comparable despite the preponderance of phonons. This makes the ratio of the light to dark current (or the signal to noise ratio) significantly exceed unity even at room temperature and enables room temperature IR detection.